This invention relates to a process and apparatus for using electromagnetic radiation for removing oxide from metal. More specifically, the invention includes using a pulsed laser to produce electromagnetic radiation having a very short pulse width, a very high pulse repetition rate and a high average power to descale the metal by vaporizing molecular layers of the oxide with each pulse.
One of the more environmentally intensive operations in the metal making industry is the wet acid pickling of a metal such as steel to remove oxide or scale formed during hot processing such as forging, rolling on a hot strip mill, or annealing. The techniques for removing this scale have changed little since the beginning of the century. Most low carbon and electrical steel strip are immersion pickled in hydrochloric acid at strip speeds of about 250 m/min. Stainless steel has a more tightly adhering scale and may require shot blasting or roller leveling to loosen or crack the scale prior to acid pickling. Additionally, stainless steel pickling requires more aggressive acids such as hydrofluoric, sulfuric, or nitric and requires longer immersion times resulting in processing line speeds for strip of about 30-100 m/min. A major motivation for improving or eliminating this type of scale removing process is the capital and environmental disposal costs of the acids associated with pickling. The annual cost associated with a single production line could be as much as $8 million dollars for disposal of hazardous acid alone. A major disadvantage of chemical descaling processes is the environmental problems related to disposal of the chemicals used for pickling.
It is known to remove or assist in the removal of scale from steel using a laser. For example, U.S. Pat. No. 4,063,063 relates to a process for descaling metal products by irradiating a metal surface with a CO.sub.2 laser beam of sufficient intensity to produce a rapid and intense local heating of an oxide film. The laser described in this patent may have a beam power up to 10 kW. However, subsequent work has revealed that it is not possible to completely remove the oxide scale using pulsed or continuous wave CO.sub.2 lasers at comparable line speeds to that achieved by acid pickling. Only an unreasonably large and costly number of lasers could achieve comparable descale rates.
Japanese patent application 2-197588 relates to a method for removing scale or rust from steel. Scale or rust on the steel is irradiated with a laser beam in a UV wavelength region of the electromagnetic spectrum such as an excimer laser beam having a 100-400 nm wavelength for .ltoreq.200 nsec pulse duration time to cause fine cracking of the scale or rust. Since the power of an excimer laser is limited to about 300 Watts and a pulse repetition rate less than 1 kHz, it also would be necessary to have an unreasonably large and costly number of excimer lasers to descale the steel at economic line speeds.
CO.sub.2, Nd:YAG, and excimer lasers represent the most common high average power industrial lasers over the wavelength range from far infrared (CO.sub.2 at 10.6 microns) to the near infrared (Nd:YAG at 1.064 microns) to the ultraviolet excimer (XeCl at 0.308 microns, KrF at 0.248 microns, and ArF at 0.193 microns). In order to obtain high descale rates, it is necessary to use very high power lasers. Many of the high average power, e.g., greater than 1 kW, commercially available lasers operate in a continuous wave (CW) mode. CW operation presents problems for scale removal due to the absorption of the incident laser beam by the plasma plume generated by the removed elemental components. This is caused by the relative long dwell times associated with CW laser processing. This has been confirmed by work done by Schluter, et al disclosed in an article entitled Descaling of Austenitic Steels by Laser Radiation, Proc. ICALEO 94, Orlando, Fla., Oct. 17-20, 1994. This same article presents data showing that even pulsed CO.sub.2 lasers are inefficient at oxide removal because the pulse duration is long enough for the incoming laser beam to interact with the created plasma plume so that some of the incoming laser energy is absorbed by this plasma thereby not removing additional oxide. The net result is a low descale rate. Wehner et al. disclose in an article entitled Ablation of Oxide Layers on Metallic Surfaces by Excimer Laser Radiation, Proc. ECLAT 90, V2, pp. 917, that short pulses, e.g., 10-250 nanoseconds, from an excimer laser are more efficient at removing the oxide layers but these lasers are only available in low average power, e.g., less than 250 W, and low repetition rates, i.e., less than 1 kHz. A simple "ball park" estimate by applicant based on covercoming the heat of vaporization of the oxide layer reveals that to remove a 5 micron thick aluminum oxide layer from one side of a 1 m wide aluminum surface moving at 31 m/min requires an average laser power of 100 kW. Although a 45 kW CO.sub.2 laser is commercially available from Trans Tec/Convergent Energy, it operates in a CW mode. Thus, even if two of these 45 kW lasers were used to cover a 1 m wide aluminum surface, the desired descale rate will not be achieved because of the plasma absorption problems associated with CW or long pulse width lasers just mentioned. To obtain this kind of power from short pulse width excimer lasers is possible only by using a large number, e.g., 400, of lasers each treating some small fraction of the desired full width material. Thus, conventional industrial lasers are not suited for economic removal of steel oxide layers.
A major disadvantage of these prior art laser descaling processes is complete scale removal from a steel strip traveling at a high speed was not possible. Alternatively, the slow strip speeds required for complete scale removal by conventional laser technology does not economically justify using this type of scale removal.
Accordingly, there remains a need for a metal oxide descaling process that does not require the use of an acid which causes an environmental disposal problem. There remains a need for a descaling process wherein an oxidized metal strip traveling at a high speed does not have to be given a shot blasting pre-treatment to loosen the scale to insure complete removal. A laser process is needed that could economically remove the scale layer. To achieve this the laser must have a large average power so that it can remove the oxide at line speeds that are comparable to that achieved with conventional acid pickling without requiring the use of an acid and/or shot blasting to assist in the removal of metal oxide from the metal strip. Another need is for the laser to be efficient in the removal of the oxide so that a minimum number of laser photons are required. To achieve this, the laser must have a very short pulse width, have a high repetition rate and have a wavelength that can be selected to give the highest scale removal rate. Another need is for the capital and operating costs of the laser to be reasonable to justify the economics of the process.